Fire requires fuel, heat, and oxygen to exist. A chemical reaction that breaks water molecules into their constituent hydrogen and oxygen is capable of setting water on fire. Potassium is a good example. A chunk of pure potassium, when tossed onto a lake, will skid across the surface, burning as it goes. It is actually the hydrogen (the fuel) being liberated from the water that is burning, and the liberated oxygen is allowing it to do so.